The Sony QX10 is not really an external lens for your phone; it’s essentially a small point-and-shoot camera that uses the phone as its screen and controller. So the real comparison is QX10/point-and-shoot vs entry-level DSLR.

A DSLR will generally win on image quality, especially in low light, because of its much larger sensor. It also gives shallower depth of field when wanted, better background blur, and much faster operation overall: autofocus, shot-to-shot response, saving images, and zoom handling are typically better.

The QX10’s strengths are portability and convenience. If you want something compact and more flexible than a phone alone, it can make sense. But once you attach it, the setup is closer in bulk to a compact camera anyway.

One community point worth noting: with a kit lens, a cheap DSLR may not produce dramatically better image quality than similarly priced point-and-shoot cameras in every situation. Still, the DSLR offers more room to grow and better performance in speed and creative control.

If your priority is best image quality and photographic capability, choose the DSLR. If your priority is portability and casual use, the QX10 may be enough.